Panels such as plywood, chipboard, oriented-strand board, and fiberboard are typically made in multistage panel presses where the layers and/or fibers are hot pressed together with a binder to produce rigid finished panels. Not only must the press platens be heated and/or cooled to activate and cure the binder, but it is standard to inject steam, which may be superheated, into the workpieces as they are being pressed and even to suck any residual steam or other vapors out of the panels before the press is opened. Such a panel press is used for the production of oriented-strand board using heat-cured phenolic-resin binders. Melamine panels and structural members having thermosetting resins are also made in such a press.
Hence it is necessary to provide each platen with a network of heating or cooling passages through which a heat-exchange liquid is passed in order to maintain the platens at the desired temperature. Furthermore each platen is provided with a network or array of steam passages that communicate through a multiplicity of orifices with the surfaces of the platens. As mentioned above, this latter network can be pressurized with steam to inject steam into the workpiece, or can be evacuated to suck steam or other vapors out of the workpiece.
In a standard system as shown in German patent 570,005 the steam passages that open via the respective orifices at the platen faces are formed as blind bores that are connected to a manifold at one edge of the platen. This system often leads to uneven distribution of steam through the workpiece, with those portions close to the manifold getting more steam than the portions remote from the manifold. Such steam passages make it very difficult to form the necessary meander passages through which the heat-exchange liquid is flowed in the platen.
A system has been proposed where grooves are cut into the faces of the platen, and then perforated bars are welded into the grooves, turning the grooves into passages open laterally through the perforations in the bars. The bars must be set perfectly flush with the platen face to produce a smooth workpiece, and installation must be essentially perfect or the system will not work. Manufacture of such a platen is therefore very complex and expensive, and the finished product is never perfectly smooth. With time, the considerable thermal deformation the platen is subject to often leads to local deformations that are pressed into the workpieces, which deformations are aggravated by the fact that the bars are often of a material with a thermal coefficient of expansion that is different from that of the platen. Furthermore the bores in the bars often fill with particles from the workpiece so that eventually the platen must be replaced or meticulously cleaned.